A conventional air-conditioner for a vehicle is disclosed in which air is sent into a passenger compartment by two fans coaxially arranged (for example, refer to Patent document 1). The air-conditioner described in Patent document 1 has a first fan and a second fan sending air by being supplied with rotation power from a drive source, and a case having a first introduction port and a second introduction port which introduce both inside air and outside air. Each of the introduction ports has an inside air introduction port and an outside air introduction port, and an opening and closing door is provided to switch the outside air introduction port and the inside air introduction port to open or close. In the case, a first air passage extending toward the first fan from the first introduction port and a second air passage extending toward the second fan from the second introduction port are defined independently from each other.
In the air-conditioner, if the opening and closing door can be positioned at an intermediate position where both inside air and outside air are introduced, the introductory rate of inside air can be reduced gradually, and the introductory rate of outside air can be increased gradually, for example, according to a change in a target value for a blow-off air temperature into the passenger compartment from low temperature to high temperature. At such a mode where both the inside air introduction port and the outside air introduction port are opened, if the speed of the vehicle is raised, a large pressure will be applied to the upstream part of the outside air introduction port. As a result, outside air may flow gradually through the inside air introduction port toward an occupant in the passenger compartment directly without being air-conditioned as a backflow.
It is known that the inside air introduction port is closed if a voltage impressed to the blower is smaller than a specified value, at such a mode where both of the inside air introduction port and the outside air introduction port are opened. When the blower voltage is higher than or equal to the specified value, outside air can be compulsorily drawn into an air conditioning unit by the blower even if the pressure loss in the air conditioning unit is large, therefore the backflow can be prevented. When the blower voltage is smaller than the specified value, the backflow may be generated because the pressure loss is large in the air conditioning unit and because a power of the blower compulsorily drawing outside air is weak (for example, refer to Patent document 2).
Art described in Patent document 2 is related with a backflow prevention in case there is only one air passage in an air-conditioner, and there is no suggestion as to a control for preventing a backflow in the case of Patent document 1 where the air-conditioner has two passages independent from each other.